1. Field of the Invention
This invention relates to a composite slab laser medium in which a doped layer is formed by laser glass containing Nd.sub.2 O.sub.3 as a laser active material and also relates to a laser device using this composite slab laser medium.
2. Description of The Related Art
In recent years, in some fields of the application of a laser (for example, in the study of an X-ray laser and of nonlinear effects), there has been a necessity for what is called a high-peak-power laser which can be repeatedly operated and have high beam quality.
In such circumstances, a glass slab laser is currently under research and development. A slab has a larger surface area for cooling than a rod has. Moreover, a slab has one-dimensional temperature and gain distributions, so that the temperature and gain distributions can be compensated by employing a zig-zag geometry as a laser geometry. Thus a slab has a technical advantage that it can realize a laser having high beam quality, the operations of which are able to be very frequently repeated.
Such a glass slab laser, however, has its limits of performance owing to the facts that the width of a slab is finite and that there is a difference between real and ideal uniformities of pumping and cooling properties thereof. In principle, it is enough for improving the performance thereof to thin and widen a slab to be employed therein. However, as the result of the thinning and widening, the mechanical strength and the precision of abrasion of surfaces of the slab are degraded. Consequently, the beam quality is lowered as well as laser oscillation becomes unstable.
To solve this problem of the glass slab laser, a composite slab has been proposed by J. L. Emmett et al. of the Lawrence Livermore National Laboratory (see J. L. Emmett, W. K. Krupke, W.R. Sooy, "The Potential of High Average Power Laser", Lawrence Livermore National Laboratory, Livermore, Calif., UCRL-53571).
The proposed composite slab is formed by bonding laser glass plates (hereunder sometimes referred to as doped layers), each of which contains active ions, to both sides of a non-doped glass plate (hereinafter sometimes referred to as non-doped layers) containing no laser active ions, respectively. The non-doped layer ensures a total thickness larger than a predetermined value for the composite slab, while the doped layers is made to be thin for the purpose of realizing a composite slab equivalent to a thin monolithic slab (namely, a composite slab having the same characteristics as a thin monolithic slab has).
Inventors of the present invention have manufactured a composite slab having a layered structure by way of trial, based on the proposition described above, by performing a welding method and further has checked the performance of the thus manufactured composite slab. As the result, it has turned out that if the thickness of the doped layer is decreased on condition that the total thickness of the composite slab is kept constant, thermal birefringence effects and thermal lens effects can be mitigated even in case where the same quantity of energy is stored and that the beam quality at the time of effecting iterative operations is increased in comparison with a monolithic slab laser. Thus it has been considered that a composite slab is utilized for repeatedly operating a glass slab laser which has a high-peak-power and can decrease a beam spread.
However, it has also turned out that the composite slab has defects in that as the doped layer becomes thinner, storage efficiency (namely, efficiency in storing energy) becomes lower, similarly as a monolithic glass laser has. Further, it has been also found that if the difference between the refractive indices of the doped-layer and of the non-doped layer is equal to or larger than 1.times.10.sup.-5 or so, a transverse mode pattern splits into three layers. This is confirmed by checking a burn pattern (i.e., a transverse mode pattern) in case of a normal oscillation. Namely, the burn pattern which is a rectangular image obtained by exposing a laser beam onto a film has three layers (i.e., a bright, dark and bright layers). This results from the facts that the doped layer is different in refractive index from the non-doped layer and that a distribution of the intensity of light on the transverse mode pattern (i.e., a cross section of the laser beam) is due to a phase difference caused by the difference in refractive index between the doped and the non-doped layers. Namely, the three layers of the transverse mode pattern originate from a difference in optical length (corresponding to the phase difference) between a laser beam incident on (or emitted from) the doped layer and another laser beam incident on (or emitted from) the non-doped layer.
Thus, it is considered that only laser beams incident on or emitted from the non-doped layer are utilized for improving a phase characteristic and raising the beam quality of the glass slab laser. This, however, results in that all of the cross section of a composite slab cannot be utilized. Consequently, this gives rise to the following drawbacks.
First, the proportion of a region, which no laser beams can pass through, of the doped layer to the whole thereof increases, with the result that the efficiency in extracting energy, which is stored in the doped layer, therefrom decreases.
Second, power extractable from the whole composite slab is limited to a low level because a fluence (i.e., energy per unit area) depends on laser damage when drawing high-peak-power therefrom.
Third, the aspect ratio of a laser beam increases to an extent undesirable for handling the laser.
Thus, it has turned out that the composite slab has not only the advantages as known when proposed by Dr. Emmett et al. (namely, the beam quality as well as a thermal-dissipation-limit average power output can be improved) but the defects as described above. Incidentally, it is still an unresolved problem how to provide a composite slab with an optimal structure for a purpose in using a glass slab laser by taking such advantages and defects into consideration.
The present invention is created to eliminate the above described drawbacks of the conventional glass slab laser.
It is, therefore, an object of the present invention to provide a composite slab laser medium which can achieve a highly efficient repetitive operation by using optimal values of a quantity of laser active ions contained in each doped layer and of a thickness of each doped layer and a ratio of the thickness of each doped layer to a thickness of the whole composite slab and can obtain laser beam provided with a high beam quality and high-peak-power by performing the repetitive oscillation.
It is another object of the present invention to provide a laser which is provided with such a laser medium.